A radio frequency (rf) lesion electrode is a device used in electrosurgery, often in neurosurgery, for destroying tissue by heating. It consists typically, as shown in FIG. 1a, of a metal shaft 1, which is insulated with coating 2 over a portion of its length, with a tip 1a that is uninsulated. The shaft 1 and tip 1a are connected to hub 1d, which is usually metal, but may be plastic, and to connector means 5 which enables 1 and 1a to be raised to an rf potential V via connection to an external rf power source 3. When inserted into the living body, and when an inactive or indifferent electrode is connected at another place on the body to carry return current back to 3, then potential V will cause rf current to flow in tissue surrounding tip 1a, this, in turn, causing the tissue to heat up and be selectively destroyed. The destruction zone around tip 1a, called a heat lesion, depends on the size of the tip, the temperature to which the tissue is raised (which is related to V), tissue characteristic, and other factors. The straight electrode like that in FIG. 1a is most common for brain surgery. Manufacturers of such electrodes are Radionics, Inc. (USA), OWL Inst. Ltd., (Canada), F. L. Fischer (W. Germany), Leksell (Sweden), and Vitatron (Netherlands). All straight elecrodes, to date, involve a fixed tip geometry i.e. uninsulated tip length 1 and tip diameter d. Often a temperature sensor is in the tip 1a, and the contact(s) for it 5 are brought out through the hub 1d to connect to external temperature monitoring circuits 7. In this way, the lesion temperature can be monitored while the heat lesion is being made.
Another type of existing straight rf lesion electrode system is shown in FIG. 1b. Here, a metal, uninsulated cannula 11 is inserted into the body, through which an rf electrode, similar to that in FIG. 1a, is inserted. When 11 is connected to ground, it becomes the indifferent electrode, and rf current will flow between it and active rf tip 1a. Such an electrode system has been used for percutaneous cordotomies (rf lesions being made in the spinal cord) for years, and are offered by Radionics, Inc. and OWL, Ltd. As shown in FIG. 1b, by varying the degree of insertion of shaft 1 into cannula 11, one can vary the extension distance x of tip 1a from the tip 11a of 11. However, in all systems to date, the tip exposure length 1 of tip 1a is predetermined and fixed, not to be varied after insertion. Manufacturers offer separate electrodes, with different tip lengths 1 and tip diameters, for different neurosurgical procedures. Radionics, the largest maker of such electrodes, in fact, offers a wide range of types such as in FIG. 1a and FIG. 1b for just that reason.
A third class of rf lesion electrodes exists to date which has a side-outlet tip extension 1c as shown in FIG. 1c. All such electrode systems prior to this invention comprise a cannula 1 which has an occluded front end 1e and a side hole opening 12, through which a flexible tip 1c emerges. Tip 1c may be fully conductive, or partially insulated, and it connects to an inner cannula 15 which telescopes through 1. Outer cannula 1 may have an uninsulated straight tip portion 1a of length 1, or it may be fully insulated. Tip 1c connects to rf potential V, and, thus, can cause rf heating of tissue off the axis of cannula 1. Thus, with a single insertion tract of cannula 1, one has the option of making a straight tip lesion, if tip 1a is uninsulated, or of making an off-axis enlargement of the lesion zone.
At present, there has been one group of researchers and two manufacturers that have reported an electrode as in FIG. 1c, and their objectives for the electrode were restricted to brain surgery. The research group was Zervas, et. al., and the companies are Radionics, Inc. (USA) and F. L. Fischer (W. Germany). Zervas, et. al. and Radionics have reported an electrode for which the entire tip 1c was uninsulated, so that its area may be varied according to extension x. The F. L. Fischer Company has an electrode in which all but the tip end of 1c is uninsulated and in which cannula 15 is insulated from, and thus isolated from, cannula 1, whereby only a portion of tip 1c is active for heating. All of these designs have involved a side-hole cannula, out of which the side extension tip 1c emerges; i.e. they have a forward looking occlusion, as represented by portion 1e in FIG. 1c, in their electrode cannula, and thus they are unable to allow emergence of a straight electrode in the forward direction. The electrode system of FIG. 1c is more versatile than those of FIGS. 1a and 1b, but it still is not as versatile as is required in many neurosurgical contexts. The dimensions of the straight axial tip 1a are fixed; i.e., length 1 and the diameter of 1a are fixed and cannot be changed once the cannula 1 has been inserted into the brain. The off-axis dimension x can be varied by varying the insertion depth x of inner hub 18b relative to outer hub 1d, as in the Zervas and Radionics designs, but there are many situations, as described below, where this does not provide the universal variability of tip dimensions and shapes which is required using a single inserted guide cannula.
It is an object of this invention to provide an electrode system whereby, with a single entrance cannula, and thus with a single entrance tract into the body, one can provide: (a) a straight electrode tip with variable tip length 1 and tip diameter as desired; or, (b) a curved electrode tip which is capable of providing a variable off-axis lesioning tip. Furthermore, the system may be adaptable also to accept both straight and off-axis stimulating and/or physiologic recording electrodes for probing the target area prior to or after lesion-making through the same entrance tract.